As a result of the rapid progress of semiconductor fabrication technology and extension of the application field for memory devices, large capacity memory devices have been developed.
Such large capacity memory devices have been developed by memory cell researchers based on a fine-process technique which doubles memory capacity in each generation. In particular, the metallization process in producing semiconductor devices is one of the important processes in the fine-process technique of the memory device.
The metallization process can be applied to form the word line connecting the gate electrodes, and to form connections interconnecting the source or drain diffusing region and other elements of the memory device.
Conventionally, the metal layer between the devices has been formed mainly by physical deposition using a sputtering process as shown in FIG. 1.
Referring to FIG. 1, a stepped-portion 1 having a predetermined pattern is formed on the semi-conductor substrate 10. A contact hole 2 opening for a metallization is then formed in the stepped-portion 1. After that, the metal layer 3 is deposited by a sputtering process on the stepped-portion 1, on the inner surface of the opening, and on the semi-conductor substrate 10 at the bottom of the opening. However, due to shadowing, cusping may form along the sides of the opening, causing a disconnection, and leaving the metal layer 3 with limitations in its use. Accordingly, degradation of the step coverage characteristic occurs in the inner wall of the contact hole.
In more detail, according to the tendency towards higher densities in semiconductor devices, it is difficult to reduce the vertical geometric size in the same ratio as that of the horizontal direction of the contact hole, resulting in an increase of the aspect ratio. As a result, because of shadowing effects by the step, it is difficult to achieve a sufficient step coverage characteristic for contact holes having a high aspect ratio. This results in the disconnection of the metal layer as shown in FIG. 1.
To solve the above problem, filling up the contact hole has been attempted in various manners. For instance, there is a selective tungsten filling up technique which fills up and levels the contact hole with tungsten before the metal layer is deposited, and there is a contact hole filling up technique using polycrystalline silicon which is excellent in the step coverage characteristic.
However, in the case of the above selective tungsten filling up technique, there are problems in that the leakage current is increased due to the interface reaction of the tungsten with the silicon substrate, and tungsten's adhesion is insufficient.
In the case of the filling up technique using polycrystalline silicon, it is difficult to maintain the contact resistance within the contact hole at a constant level because the polycrystalline silicon must be changed into the conductor by an ion implantation. Further, the control for the amount of the implanted ion is difficult.